This invention relates generally to the purification of vinyl olefins and more particularly to the selective removal of branched chain olefin and especially vinylidene olefin impurities by converting them to oligomers which are easily separated from the vinyl olefins.
Olefin mixtures containing vinyl, vinylidene and internal olefins of similar carbon number are difficult to separate by distillation based solely on the olefin type because vinyl, vinylidene and internal olefins having the same carbon number boil very close together. This is generally the case when the olefins are made by a process capable of producing all three types of olefins. For example, the ethylene chain growth process using triethylaluminum followed by olefin displacement as practiced commercially can produce olefins containing from 4 to up to 30 or more carbon atoms. The olefin product is mainly vinyl olefins, i.e. R--CH.dbd.CH.sub.2 wherein R is an aliphatic hydrocarbon group, but it also contains lesser amounts of internal olefins, and vinylidene olefins.
In some uses the vinylidene olefin content of olefin mixtures is not detrimental. However, in some uses, the presence of even a few percent of vinylidene olefin decreases the value of the olefin mixture. For example, detergents can be made by reacting olefin mixtures with hydrogen sulfide to add hydrogen sulfide to the double bond forming a mercaptan. These in turn can be oxidized to form sulfonic acids which when converted to their salts are effective detergents. However, vinylidene olefins react with hydrogen sulfide to form tertiary mercaptans which are very difficult to oxidize to sulfonic acids. Thus a need exists for a process for separating vinylidene olefins from a mixture containing vinyl, vinylidene and internal olefins which mixtures cannot be readily separated by distillation. The process also has to remove the vinylidene olefins without converting a substantial amount of vinyl olefins to undesirable side products such as internal olefins.
It has now been discovered that olefin mixtures containing vinyl, vinylidene and internal olefins can be purified by reacting the mixture using a BF.sub.3 -phosphorus acid catalyst system in a manner so as to preferentially dimerize the vinylidene olefins such that the vinylidene olefin content can be reduced to below detectable levels (less than 0.1 mole percent). The dimerized product is not only easily separated from the product mixture by distillation but the product is a useful synthetic oil such that even when significant amounts of vinylidene olefins must be removed, the purification process is very economical.